The present invention relates generally to cooling systems for rotating electrical machines, and more specifically to a rotating heat pipe for removing heat energy from the rotor of an electric motor, an electric generator or the like.
Electrical systems in military and commercial aircraft typically include electric motors and generators having permanent magnet rotors which generate large amounts of heat during operation. Present methods for cooling the rotors include convective air (generally bleed air from the aircraft engine compressor) and oil circulation through a passageway inside the rotor shaft, neither of which are optimum solutions. Air is not effective as a coolant because of its poor heat transfer and low heat capacity. Plus, using bleed air may reduce overall engine performance. Oil circulation causes viscous frictional losses.
The prior art has investigated using rotating heat pipes to transport heat out of high speed rotating machines. In this concept, a rotating heat pipe is made an integral part of a rotor shaft. The rotor shaft is made hollow, filled with a suitable working fluid and sealed. During rotation, the fluid is uniformly distributed over the inner wall of the shaft. Heat is transferred from the rotor through the outer wall of an evaporator section of the rotor shaft to vaporize the fluid inside the rotor shaft. Pushed by the vapor pressure of the vaporized fluid, the vapor flows to a colder condenser section of the shaft away from the rotor and condenses on the tapered inside wall of the condenser section. The centrifugal force from the rotating shaft causes the condensate to flow along a tapered inner wall back to the evaporator section in thermal contact with the rotor, thus completing the heat transport cycle. U.S. Pat. No. 5,283,488 to two of the present inventors introduced the rotofin concept as an improvement to the basic integral heat pipe rotor shaft concept to better acquire the heat energy in the rotor and transfer it to the rotating heat pipe. As just described, the rotating heat pipe will then transfer the acquired heat energy from its evaporator section in thermal contact with the rotor to its condenser section. The last remaining problem is to remove the heat from the condenser section.
Thus it is seen that there is a need for a system for removing heat energy from the condenser section of a rotating heat pipe which is an integral part of the rotor shaft of a high speed electric motor or generator.
It is, therefore, a principal object of the present invention to provide a system for removing heat energy from the condenser section of an integral heat pipe rotor shaft of a high speed electric machine such as an electric motor or generator.
It is a feature of the present invention that it provides a very compact integrated system for removing heat from the rotor of a high speed, high power rotating electrical machine.
It is an advantage of the present invention that it can safely use as a liquid coolant any liquid located near the mounting location of the electrical machine. For example, it can safely use aircraft jet fuel without risk of igniting the jet fuel.
It is another advantage of the present invention that air cooling would require 1000 times the volume of a liquid coolant to remove the same amount of heat from the condenser section of an integral heat pipe rotor shaft.
It is a further advantage of the present invention that it permits a far greater volume of liquid coolant to be used for cooling the rotor of an electrical machine than is possible by direct cooling of the rotor with a liquid coolant.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.